Illuminating apparatus and ballast circuit

ABSTRACT

One or more gaseous discharge lamps which have a negative resistance characteristic are connected to a ballast circuit which includes a transformer having primary and secondary coils and a core having a portion of reduced cross-sectional area. The core also includes a high-reluctance shunt which is arranged to cooperate with the portion of reduced cross-sectional area to provide a path for a portion of the magnetic flux linking the windings of the primary coil that does not link the windings of the secondary coil. This construction diminishes the tendency of the current flow in the secondary coil to increase as the resistance of the lamp decreases. In addition, it improves voltage regulation and the crest factor of the current.

United States Patent De Leon [4 1 Aug. 15, 1972 [54] ILLUMINATINGAPPARATUS AND Primary Examiner-Roy Lake BALLAST CIRCUIT AssistantExaminer-Palmer C. Demeo [72] Invemor, Luis F De Leon 1012 J EscodaAttorney-Brumbaugh, Graves, Donohue & Raymond Llanes, Manila,Philippines 57] ABSTRACT [22] filed: 1970 One or more gaseous dischargelamps which have a [21] Appl. No.: 68,171 negative resistancecharacteristic are connected to a ballast circuit which includes atransformer having primary and secondary coils and a core having aportion 315/ of reduced cross-sectional area. The core also includes ahighqeluctance shunt which is arranged to [58] Field of Search..315/276, 278, 282, 239, 323 cooperate with the portion of reducedcmwsecfional area to provide a path for a portion of the magnetic [56]References C'ted flux linking the windings of the primary coil that doesU E STATES PATENTS not link tlcile windlingsflof thedsecondzrilhcoil.This; (Eonstruction imlms es e en ency o e curren ow 2,810,100 10/ 1957Strecker ..315/278 X in the secondary coil to increase as the resistanceof 2,858,479 10/1958 Sola ..3l5/239 X the lamp decreases. In addition itimproves voltage lvi i regulation and the crest factor of the current.oer ens 10 Claims, 6 Drawing Figures PATENTEDAm; 15 I972 3.684.921

sum 1 0f 2 INVENTOR.

LUIS F. DE LEON BY); 4, 4&

and?

his AT ORNEYS PATENTEmuc 15 I972 3.684.921

sum 2 or 2 a FIG. 6

INVENTOR.

LUIS F. DE LEON BY "1 g 74 i JML a/f his A TTORNEYS ILLUMINATINGAPPARATUS AND BALLAST CIRCUIT BACKGROUND OF THE INVENTION This inventionrelates to illuminating apparatus of the gaseous discharge type, andmore particularly to illuminating apparatus of the gaseous dischargetype in which the lamps have a negative resistance characteristic thatrequires the use of a ballast circuit.

It is well known that gaseous discharge lamps generally have a negativeresistance characteristic. By this it is meant that as the current flowthrough the lamp increases the resistance of the lamp decreases. Thus,these lamps present a relatively high resistance when they are firstturned on and a relatively high voltage must, therefore, be applied tostart the lamps. However, after the current begins to flow between thefilaments of the lamp, the resistance falls off and the currentcontinues to increase. If the appropriate circuit elements are notincluded in combination with the lamp to provide an impedance to theflow of current, the current will increase until the lamp reaches whatis known as a runaway condition and burns out. Accordingly, it isconventional to employ a ballast circuit which provides this neededimpedance in combination with such lamps.

Conventional ballast circuits usually include a capacitor arranged inseries with the lamp to provide the required impedance. Conventionalballast circuits also include a transformer to provide the voltagenecessary to start and operate the lamp from an availablealternating-current power supply. The total impedance presented by theballast circuit is, of course, a combination of the inductive reactanceof the transformer and the capacitive reactance of the capacitor.Ballast circuits of this type are sometimes combined with two gaseousdischarge lamps which are arranged for sequence starting and seriesoperating.

In addition to the capacitor mentioned above which is connected inseries with both lamps across the secondary coil of the transformer, astarting capacitor is often connected in parallel with one of the twolamps. When power is first supplied to the transformer, the startingcapacitor provides a path by which the current bypasses one of the twolamps and the available voltage is impressed upon the other lamp whichis started first. After this lamp has started and its resistance drops,the voltage drop across the unstarted lamp becomes high enough to startit, and its resistance drops to the extent that the current flow throughthe starting capacitor is insubstantial.

Because the brightness of gaseous discharge lamps is dependent, to aconsiderable extent, upon the voltage supplied across their filaments,it is highly desirable if voltage regulation can be built into theballast circuit. The effect of the apparatus upon the power factor isanother important consideration in designing a ballast circuit. Thecrest factor of the current which flows through the lamps is anadditional operating parameter to be considered when evaluating theperformance of a ballast circuit, primarily because of its effect on theoperating life of a lamp.

It has previously been proposed by this inventor that conventionalballast circuits could be improved by forming a slot or hole in thecenter portion of the transformer core surrounded by the secondary coil.This modification of the transformer brings about a desirable saturationof the portion of the core which has a reduced cross-sectional area andthereby improves the operation of the ballast circuit. This earlierinvention is explained in detail in Pat. No. 4972 issued by the Republicof the Philippines on May 1 l, 1970.

To fully appreciate the problems involved in the design of ballastcircuits it must be understood that they are utilized in connection withlamps, such as conventional florescent lamps used in homes and offices,which must be designed for high production at a minimum cost. Ittherefore follows that small sophisticated improvements in their designwhich enable them to be produced at a lower cost utilizing lessexpensive components are of great significance SUMMARY OF THE INVENTIONThis invention consists of an important improvement in the design ofballast circuits employing the board principle of reducing thecross-sectional area of a portion of the transformer core. A transformeris employed which has a primary coil adapted to be connected to analtemating-current power supply, a secondary coil adapted to beconnected to one or more gaseous discharge lamps, and a core with whichthe primary and secondary coils are magnetically associated. Ahigh-reluctance shunt is arranged to cooperate with the portion of thecore which is of reduced cross-sectional area and is positioned toprovide a path for a portion of the magnetic flux linking the windingsof at least a part of the primary coil that does not link the windingsof a substantial part of the secondary coil. This construction reducesthe tendency of the current flow in the secondary coil to increase asthe resistance of the lamp decreases. It also improves the voltageregulation inherent in the ballast circuit.

Preferably, the portion of the transformer core which is of reducedcross-sectional area is disposed adjacent to one end of the secondarycoil. It is formed by at least one indentation in the centerpiece of thecore. The high-reluctance shunt is formed by a protruding portionattached to a side structure of the core which projects part way intothe indentation.

In a preferred embodiment of the invention, the transformer core is ofthe shell type in which the two opposite ends of the centerpiece aboutwhich the primary and secondary coils are wound are connected by a sidestructure. In this embodiment, the portion of the core of reducedcross-sectional area is formed by two indentations oppositely disposedon the sides of the centerpiece and the shunt is formed by twoprotruding portions on the side structure, one of which projects partway into each of the two indentations.

The entire core is made of a plurality of laminations arranged to form astack. This is a conventional arrangement which is desirable because itminimizes eddy current losses. An important feature of this transformeris that, because the protruding portions of the side structure coincidewith the indentations in the centerpiece, the laminations of thecenterpiece and the side structure can conveniently be made by onepunching operation which produces a minimum of scrap.

The ballast circuit of this invention can be combined with first andsecond gaseous discharge lamps each having a negative resistancecharacteristic. An operating capacitor is connected in series with thesecondary coil and the first and second lamps to provide an impedancewhereby the tendency of the current to increase as the resistance oflamps decreases is further diminished. A starting capacitor is connectedin series with the first lamp and the secondary coil and in parallelwith the second lamp whereby, when the apparatus is first connected tothe power supply, the current passes through the starting capacitor andbypasses the second lamp until the first lamp is started. The secondlamp is then started after the resistance of the first lamp hasdiminished.

The entire ballast circuit may be encapsulated in a block of materialand a temperature sensing means for disconnecting the primary coil ofthe transformer from the power supply when the temperature rises above apredetermined level may be included. This will protect the device andthe surrounding environment in the event that excessive heat is createddue to a malfunction of the apparatus.

The ballast circuit described above has been. found to be particularlysuitable for use in combination with florescent lamps. Its voltageregulating effect, its crest factor, and its effect on the power factorhave been found to be quite desirable in comparison to previously knownballast circuit constructions. Moreover, when the transformer of thisinvention is employed it is possible to utilize a smaller operatingcapacitor than would otherwise be required to provide the necessaryimpedance and to prevent a runaway condition of the lamps from beingreached. The functionally desirable shape of the laminations of thecenterpiece and side structure of the transformer coil facilitates themanufacture of these sheets of material by a single punching operationwhich produces a minimum of scrap.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding ofthe invention, reference may be made to the following detaileddescription taken in conjunction with the accompanying figures of thedrawings in which:

FIG. 1 is a partially schematic cross-sectional view of a transformersuitable for use in a first embodiment of the invention;

FIG. 2 is a cross-sectional view taken along the line 2-2 of thetransformer of FIG. 1;

FIG. 3 is a schematic view of an illuminating apparatus including aballast circuit constructed in accordance with the invention andemploying the transformer of FIG. 1;

FIG. 4 is a partially schematic cross-sectional view of a transformersuitable for use in a second embodiment of the invention;

FIG. 5 is a schematic diagram of an illuminating apparatus including aballast circuit constructed in accordance with the invention andemploying the transformer of FIG. 4; and

FIG. 6 is a graph showing a typical wave form of the current that flowsthrough the lamps of the circuit shown in FIGS. 3 and 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a partiallyschematic cross-sectional view of an isolation transformer 10 suitablefor use in a ballast circuit constructed in accordance with theinvention. The transformer 10 includes a core 12 fonned by a centerpiece14 and a side structure 16. The side structure 16 is made of a pair ofsymmetrical side pieces 18 and 20 which connect the ends of thecenterpiece 14. As can best be seen in the cross-sectional view of FIG.2, each portion of the core 12 is made up of a plurality of laminations22 punched from a sheet of magnetic material and arranged to form astack.

The transformer 10 is an isolation transformer which includes a primarywinding 24 and a separate secondary winding 26. The centerpiece 14 ofthe transformer core 12 includes a portion 28 which is of reducedcrosssectional area. The portion 28 is located between two indentations30 which are disposed on opposite sides of the centerpiece 14. The sidestructure 16 of the core 12 carries a pair of protruding portions 32 oneof which projects part way into each of the indentations 30. Theprotruding portions 32 are dimensioned so as to leave a non-magnetic gapbetween the protruding portions 32 and the centerpiece 14. Theprotruding portions 32 cooperate with the indentations 30 to form a pairof high-reluctance shunts 34 disposed between the primary coil 24 andthe secondary coil 26.

The transformer 10 includes a pair of leads 36 by which the primary coil24 is adapted to be connected to an alternating-current power supply. Apair of leads 38 are provided for connecting the secondary coil 26 to alamp. A plurality of leads 40 are provided for connecting a pair ofauxiliary secondary coils 42 (not shown in FIG. 1) to the filaments of agaseous discharge lamp of the continuously heated filament type.

FIG. 3 shows a schematic diagram of the transformer of FIG. I arrangedin combination with a gaseous discharge lamp 44 which may be of thefluorescent type having a negative resistance characteristic. The lamp44 is connected by the leads 38 to the secondary coil 26. It isconnected in series with an operating capacitor 46. The lamp 44 includesa pair of continuously heated filaments 48 each of which is connected toone of the auxiliary secondary coils 42 by the leads 40.

The high-reluctance shunts 34 provide paths for part of the fluxgenerated by the primary coil 24 which does not link the windings of thesecondary coil 26. Although this flux is intentionally permitted toavoid the secondary coil 26, it may be referred to as leakage flux. Asthe current increase and the magnetic flux in the core 12 of thetransformer 10 increases, the portion 28 of the centerpiece 14 which isof reduced cross-sectional area will tend to become saturated. As thesaturation of the portion 28 increases the amount of leakage fluxfollowing the path which does not link the windings of the secondarycoil 26 will increase. This tends to limit the current induced in thesecondary coil 26.

It will be recalled that the lamp 44 has a negative resistancecharacteristic, and, therefore, once started it will tend to draw alarger and larger current until it reaches a runaway condition if animpedance is not provided to limit the current supplied to the lamp 44.The operating capacitor 46 provides part of this needed impedance.However, because of the current limiting effect of the high-reluctanceshunts 34 on the secondary coil 26, the capacitor 46 may have asubstantially smaller farad capacity than would be necessary if theshunts 34 were not provided and the centerpiece 14 of the transformercore 12 were of constant cross-sectional area. Thus, the tendency of thecurrent flow in the secondary coil 26 to increase as the resistance ofthe lamp 44 decreases is diminished by the shunts 34. Accordingly, theapparatus shown in FIG. 3 can be manufactured at a lower cost because ofthe reduction in the size of the capacitor 46.

It should be noted that the presence of the highreluctance shunts 34also provides for improved voltage regulation in the transformer 10. Avariation in the voltage applied across the leads 36 of the primary coil24 will produce a proportionately smaller variation in the voltageinduced in the secondary coil 26, partly because of the construction ofthe core 12 and the shunts 34.

Another advantage of the transformer construction described above isthat the crest factor of the current that flows through lamp 44 isminimized and may be made substantially lower than the crest factorsassociated with previously known transformers for use in ballastcircuits. The crest factor is defined as the ratio of the maximumcurrent flow which occurs during a cycle of the alternating-currentpassing through the lamp 44 to the root-mean-square value of thatcurrent. Hence, a current which flows in the form of a relatively fiatwave will have a low crest factor as compared to a current which flowsas a wave that rises sharply in the form of a spike. It has been foundthat current having a low crest factor contributes to long life andstable operation of gaseous discharge lamps. A graphical representationof an oscilloscope tracing produced by the current flowing through thecircuit of FIG. 3 is depicted in FIG. 6. The precise shape of the waveform is, of course, dependent upon the particular circuit componentsselected and this graph is merely illustrative of a typical wave form.The ordinate of the graph of FIG. 6 represents current amplitude and theabscissa represents time. No scale of numerical values is given inconjunction with this graph because only the shape of the wave is ofinterest here.

Consistent with the objective of minimizing the manufacturing cost ofthe apparatus, the laminations 22 of which the core 12 is made areshaped so that they conveniently fit together and do not overlap. Forthis reason, the side members 18 and 20 and the centerpiece 14 can bepunched together from a single sheet of magnetic material therebyaffecting a considerable savings.

It is desirable to enclose all of the components of the ballast circuitshown in FIG. 3 in a block of encapsulating material. A section of suchan insulating block 85 is represented diagrammatically in broken linesin FIG. 3. A temperature responsive means for disconnecting the primarycoil 24 from the power supply in the event of a rise in the temperatureof the block of material above a predetermined level due to amalfunction of the apparatus can be included in the circuit.

A second embodiment of the invention is shown in FIGS. 4 and 5. FIG. 4shows a partially schematic crosssectional view of a transformer 52having a core 54.

This core 54 is of the same design as the core 12 shown in FIG. 1. Theparts of the core 54 are, therefore, designated by the same numbers asthe corresponding parts of the core 12 of the transformer 10 and are notseparately described here.

The transformer 52 differs from the transformer 10 in that it is anautotransformer instead of an isolation transformer. Thus, there is, ineffect, only one transformer coil 56 which is divided into two parts 58and 60. A pair of high-reluctance shunts 61 formed by the projectingportions 32 and the indentations 30 of the transformer core 54 separatethese two sections 58 and of the coil 56. The entire coil 56 functionsas the secondary coil of the transformer 52. The section 58 of the coil56 also functions as the primary coil of the transformer 52.

FIG. 5 is a schematic circuit diagram of the transformer 52 arranged incombination with a first lamp 62 and a second lamp 64. Each of the lamps62 and 64 are of the gaseous discharge type and have a negativeresistance characteristic. They may be conventional florescent lamps.The primary coil 58 is adapted for connection to an alternating-currentpower supply by a pair of leads 66. An operating capacitor 68, which isconnected in parallel with a leakage resistor 70, is connected in serieswith the first lamp 62, the second lamp 64, and the secondary coil 56. Astarting capacitor 72 is connected in series with the first lamp 62 andthe secondary coil 56 and in parallel with the second lamp 64.

The operating capacitor 68 performs the same function as the operatingcapacitor 46 shown in FIG. 3. It provides an impedance in series withthe lamps 62 and 64 to prevent them from reaching a runaway condition astheir resistance decreases. The function of the starting capacitor 72 isto allow the current to bypass the second lamp 64 when the primary coil58 is first connected to the power supply. Because the current flowsthrough the starting capacitor 72 instead of the second lamp 64, thevoltage applied across the first lamp 62 is higher than it would be ifthe current from the secondary coil 56 passed through the two lamps 62and 64 in series. Thus, a relatively high starting voltage is applied tothe first lamp 62 and, after the lamp 62 has started and its resistancehas decreased, the voltage drop across the starting capacitor 62 beginsto rise until the second lamp 64, which is connected in parallel withthe starting capacitor 72, is also started. When both lamps 62 and 64are in a stable operating condition, the current passing through thecapacitor 72 is insubstantial. Thus, because the operating capacitor 68and the starting capacitor 72 are employed in combination, the lamps 62and 64 are started sequentially but are operated in series.

The lamps 62 and 64 are both of the continuously heated filament type.Accordingly, these lamps each include a pair of filaments 74 that arecontinuously supplied with current from a plurality of auxiliarysecondary coils 76. The auxiliary coils 76 are connected to thefilaments 74 by a plurality of leads 78. The second coil 56 is connectedto the lamps 62 and 64 by a pair of leads 80. A temperature responsivemeans 82 for disconnecting the primary coil 58 from the power supply inthe event of a rise in the temperature above a predetermined level isprovided as in the apparatus of FIG. 3.

As in the embodiment shown in FIG. 3, the protruding portions 32 of thetransformer core side structure 16 cooperate with the indentations 30 toform a pair of high-reluctance shunts which are designated 61 in FIGS. 4and 5. These high-reluctance shunts 61 provide a path for the magneticflux linking the windings of the primary coil 58 that does not link thewindings of a substantial part 60 of the secondary coil 56 whereby thetendency of the current flow in the secondary coil 56 to increase as theresistance of the lamps 62 and 64 decreases is diminished. Moreover, asexplained in reference to the embodiment of FIG. 3, the presence of thehigh reluctance shunts 61 in combination with the portion 28 of the core54 which is of reduced cross-sectional area improves the voltageregulation which is inherent in the construction of the transformer 52.Another inherent advantage to this construction is that the crestfeature of the wave is of a lower value than can be obtained utilizingpreviously known concepts of transformer design.

It will be clear from the above description that the invention embodiedin the apparatus of FIGS. 3 and 5 represents an advance of considerablesignificance in the art of illumination by gaseous discharge lamps.Because the design of the apparatus minimizes manufacturing costs it isparticularly suitable for use in conjunction with florescent lamps whichare associated with high volume production and, therefore, place greatemphasis on the minimization of the cost of ballast circuit components.

It will be obvious to those skilled in the art that the above-describedembodiments are meant to be merely exemplary and that they aresusceptible of modification and variation without departing from thespirit and scope of the invention. Therefore, the invention is notdeemed to be limited except as defined by the appended claims.

I claim:

1. A ballast transformer for use in a ballast circuit for starting andoperating a gaseous discharge lamp having a negative resistancecharacteristic; the transformer comprising a primary coil and asecondary coil, a core defining a magnetic circuit linking the primaryand secondary coils, the core including a centerpiece encircled by thecoils and at least one said member which completes a magnetic circuitlinking the primary and secondary coils through the centerpiece andthrough the side member from one end of the centerpiece to another endof the centerpiece, and means for preventing an extreme increase in fluxlinking the substantial secondary section upon starting of a gaseouslamp connected thereto, said means for preventing including a saturablenarrowed section of the centerpiece intermediate the primary coil and atleast a substantial section of the secondary coil, and a high-reluctanceshunt near said narrowed section located to divert flux from thesaturated narrowed section and through the shunt and forming a leakagemagnetic path from the centerpiece to the side member that does not linksaid substantial secondary section, the magnetic circuit through thecenterpiece and through the side member being, absent saturation, oflower reluctance than the leakage magnetic path through the centerpiece,the shunt, and the side member, whereby a greater portion of flux isdiverted through the high-reluctance shunt after starting of a gaseousdischarge lamp connected with said 4 secondary coil causing saturationin the narrowed section of the centerpiece.

2. The ballast transformer of claim 1, said narrowed section of thecenterpiece being formed by at least one indentation in said centerpieceand said shunt being formed by at least one protruding portion attachedto said side member which projects part way into said indentationforming said air gap at said indentation entirely separating theprotruding portions and the centerpiece at the indentation.

3. The ballast transformer of claim 2 wherein there are two of saidindentations disposed along opposite sides of said centerpiece and thereare two protruding portions each attached to a side member eachprojecting part way into one of said indentations.

4. A ballast circuit including the ballast transformer of claim 3,further comprising a block of material in which said transformer isencapsulated and a temperature responsive means encapsulated in saidblock for disconnecting said primary coil from a power supply when thetemperature rises above a predetermined level. V

5. The ballast transformer of claim 1, wherein said transformer is anisolation transformer and said narrowed section of the centerpiece isdisposed between said primary coil and the entire secondary coil.

6. The ballast transformer of claim 1, wherein said transformer is anautotransformer and said narrowed section of the centerpiece is disposedadjacent one end of said primary coil.

7. The ballast transformer according to claim 1, wherein saidhigh-reluctance shunt includes a magnetic leg and an air gapintermediate and entirely separating the centerpiece and the side membernear the narrowed section, and said magnetic circuit through thecenterpiece and the side member being continuous throughout.

8. A ballast circuit including a transformer according to claim 1,output circuit means electrically connected with said secondaryconnection to at least one negative resistance characteristic gaseousdischarge lamp, and means for connecting the primary coil of thetransformer to a predetermined input voltage, the narrowed centerpiecesection of the transformer core having a reduced cross-section narrowedto saturate when the input voltage is applied to the primary coil andthe output circuit means begins conducting increasing current through aconnected lamp.

9. An illuminating apparatus comprising first and second gaseousdischarge lamps each having a negative resistance characteristic and aballast circuit to which said lamps are connected for sequence startingand series operating wherein said ballast circuit comprises atransformer having a primary coil adapted to be connected to analternating-current power supply, a secondary coil connected to supplycurrent to said first and second lamps, and a magnetic core, said corebeing of the shell type having a centerpiece about which said primaryand secondary coils are wound and a side structure connecting twoopposite ends of said centerpiece, said centerpiece having a portion ofreduced cross-sectional area formed by two indentations disposed onopposite sides of said centerpiece, saturable upon an increase incurrent in the secondary coil, and said said structure having twoprotruding portions which project part way into said indentations toform high-reluctance shunts which are positioned to provide a path for aportion of the magnetic flux linking the windings of at least a part ofsaid primary coil that does not link the windings of a substantial partof said secondary coil, the saturable portion of reduced cross-sectionalarea and the shunts comprising means for diverting an increasingproportion of the flux linking the primary coil away from thesubstantial part of the secondary whereby the tendency of the currentflow in the secondary coil to increase as the resistance of the lampsdecreases is diminished and whereby the voltage regulation of saidballast circuit is improved.

1. A ballast transformer for use in a ballast circuit for starting andoperating a gaseous discharge lamp having a negative resistancecharacteristic; the transformer comprising a primary coil and asecondary coil, a core defining a magnetic circuit linking the primaryand secondary coils, the core including a centerpiece encircled by thecoils and at least one said member which completes a magnetic circuitlinking the primary and secondary coils through the centerpiece andthrough the side member from one end of the centerpiece to another endof the centerpiece, and means for preventing an extreme increase in fluxlinking the substantial secondary section upon starting of a gaseouslamp connected thereto, said means for preventing including a saturablenarrowed section of the centerpiece intermediate the primary coil and atleast a substantial section of the secondary coil, and a high-reluctanceshunt near said narrowed section located to divert flux from thesaturated narrowed section and through the shunt and forming a leakagemagnetic path from the centerpiece to the side member that does Not linksaid substantial secondary section, the magnetic circuit through thecenterpiece and through the side member being, absent saturation, oflower reluctance than the leakage magnetic path through the centerpiece,the shunt, and the side member, whereby a greater portion of flux isdiverted through the high-reluctance shunt after starting of a gaseousdischarge lamp connected with said secondary coil causing saturation inthe narrowed section of the centerpiece.
 2. The ballast transformer ofclaim 1, said narrowed section of the centerpiece being formed by atleast one indentation in said centerpiece and said shunt being formed byat least one protruding portion attached to said side member whichprojects part way into said indentation forming said air gap at saidindentation entirely separating the protruding portions and thecenterpiece at the indentation.
 3. The ballast transformer of claim 2wherein there are two of said indentations disposed along opposite sidesof said centerpiece and there are two protruding portions each attachedto a side member each projecting part way into one of said indentations.4. A ballast circuit including the ballast transformer of claim 3,further comprising a block of material in which said transformer isencapsulated and a temperature responsive means encapsulated in saidblock for disconnecting said primary coil from a power supply when thetemperature rises above a predetermined level.
 5. The ballasttransformer of claim 1, wherein said transformer is an isolationtransformer and said narrowed section of the centerpiece is disposedbetween said primary coil and the entire secondary coil.
 6. The ballasttransformer of claim 1, wherein said transformer is an autotransformerand said narrowed section of the centerpiece is disposed adjacent oneend of said primary coil.
 7. The ballast transformer according to claim1, wherein said high-reluctance shunt includes a magnetic leg and an airgap intermediate and entirely separating the centerpiece and the sidemember near the narrowed section, and said magnetic circuit through thecenterpiece and the side member being continuous throughout.
 8. Aballast circuit including a transformer according to claim 1, outputcircuit means electrically connected with said secondary connection toat least one negative resistance characteristic gaseous discharge lamp,and means for connecting the primary coil of the transformer to apredetermined input voltage, the narrowed centerpiece section of thetransformer core having a reduced cross-section narrowed to saturatewhen the input voltage is applied to the primary coil and the outputcircuit means begins conducting increasing current through a connectedlamp.
 9. An illuminating apparatus comprising first and second gaseousdischarge lamps each having a negative resistance characteristic and aballast circuit to which said lamps are connected for sequence startingand series operating wherein said ballast circuit comprises atransformer having a primary coil adapted to be connected to analternating-current power supply, a secondary coil connected to supplycurrent to said first and second lamps, and a magnetic core, said corebeing of the shell type having a centerpiece about which said primaryand secondary coils are wound and a side structure connecting twoopposite ends of said centerpiece, said centerpiece having a portion ofreduced cross-sectional area formed by two indentations disposed onopposite sides of said centerpiece, saturable upon an increase incurrent in the secondary coil, and said said structure having twoprotruding portions which project part way into said indentations toform high-reluctance shunts which are positioned to provide a path for aportion of the magnetic flux linking the windings of at least a part ofsaid primary coil that does not link the windings of a substantial partof said secondary coil, the saturable portion of reduced cross-sectionalarea and the shunts comprising means for diverting an increasingproporTion of the flux linking the primary coil away from thesubstantial part of the secondary whereby the tendency of the currentflow in the secondary coil to increase as the resistance of the lampsdecreases is diminished and whereby the voltage regulation of saidballast circuit is improved.
 10. The apparatus of claim 9 in which thetransformer core centerpiece and said structure form a continuousmagnetic path linking both the entire primary coil and the entiresecondary coil, the centerpiece, high reluctance shunt, and outerstructure forming a second magnetic path of higher reluctance than thecontinuous path and having air gaps where the protruding portionsproject part way into said indentations, the protruding portions beingentirely spaced from and unconnected with the indentations where theprotruding portions project.